This invention relates to the construction and operation of pressure vessels and, particularly, the construction and operation of hot isostatic presses.
Isostatic presses are used to compress powdered material contained in a mold in order to form a solid article. In the operation of isostatic presses, it is not uncommon to employ pressures as high as 50,000 psi. Frequently, it is desired to isostaticly press a powdered metal. As those skilled in this art appreciate, when isostaticly pressing a powdered metal mold it is often desirable that the powdered metal be pre-heated and that the pressing process occur at an elevated temperature. Presses for conducting such processes are referred to as hot isostatic presses and generally include heating elements for maintaining a high temperature within the press.
In a hot isostatic press, it is obviously desirable that heat generated by the heating elements be employed to maintain at a high temperature the article to be compressed. Stated otherwise, to the extent possible it is desirable to isolate the heating elements from the walls of the press thereby avoiding heat loss. In an effort to thermally isolate the heating elements from the walls of a hot isostatic press, it is conventional to employ one or more heat insulating sheaths or mantles. Such mantles are generally in the form of cylindrical shells which surround the heating elements and are interposed between the heating elements and the walls of the press. For a number of reasons, the structural strength of such a sheath or mantle is generally not substantial. Additionally, the size of such mantles may be considerable, for example a typical mantle may have an inner diameter of 2 feet and a length of 5 or 6 feet.
Because of the size and construction of a mantle, it is particularly susceptible to failure as a result of any pressure differences which may exist across the mantle.
In prior art hot isostatic presses, a single port is provided for pressurizing and depressurizing. Such a single port will generally provide fluid communication with the interior of the press, on one side of the mantle. Generally, means are then provided for insuring fluid communication with the other side of the mantle. For example, in U.S. Pat. No. 3,695,597, a pressurization/depressurization port is provided and is in communication with the annular space defined by the wall of the press and the outer surface of a heat insulating sheath or mantle. The heat insulating sheath, in this construction, is secured to the upper part of the press and the lower part of the sheath terminates at a point above the bottom of the press whereby the pressurizing medium, which in the case of the hot isostatic pressing process is generally a gas, may flow around the bottom of the sheath into the interior or working area of the press.
Although prior art hot isostatic presses generally provide some means for fluid communication between the outside and the inside of the mantle, nevertheless it has been found that during severe pressure changes, for example during depressurization, a pressure differential may be created across the mantle thus causing a structural failure of the mantle. This invention provides a method and an apparatus, including a novel sub-combination, which prevents the occurrence of such a pressure differential thus insuring that a structural failure of the mantle will not occur during either pressurization or depressurization of an isostatic press.